Zest one lemon with a microplane grater. Cut all threee lemons in half; juice. In a medium sauce pan over medium heat, whisk together the lemon zest, lemon juice, eggs and sugar. Whisk constantly; do NOT boil. Once the mixture is hot, turn the heat to low and whisk in the butter, 1 tablespoon at a time. Continue cooking on low for 6 minutes until the lemon glaze has thickened. Remove from heat and let cool.